Oxide glasses capable of transmitting radiations into the infrared region of the electromagnetic spectrum are well known in the art. To illustrate:
U.S. Pat. No. 3,723,141 (Dumbaugh, Jr.) describes glasses which transmit infrared radiations at wavelengths longer than six microns consisting essentially, in weight percent, of
______________________________________ PbO 10-75 BaO 2-25 Bi.sub.2 O.sub.3 10-85 ZnO 1-10 PbO + Bi.sub.2 O.sub.3 .gtoreq.60 Sio.sub.2 + B.sub.2 O.sub.3 + P.sub.2 O.sub.5 .ltoreq.1 ______________________________________
with, optionally, up to 10% individually and up to 20% total of an oxide selected from the group of As.sub.2 O.sub.3, CaO, CdO, GeO.sub.2, HgO, Sb.sub.2 O.sub.3, SrO, TiO.sub.2, and alkali metal oxides.
U.S. Pat. No. 3,837,868 (Berleue et al.) reports glasses consisting essentially, in weight percent, of
______________________________________ Bi.sub.2 O.sub.3 8-80 PbO + CdO .gtoreq.5 PbO 0-57 Fe.sub.2 O.sub.3 5-32.5 CdO 0-32 ______________________________________
with, optionally, up to 15% total of the following oxides in the indicated proportions consisting of up to 7.5% BaO and/or ZnO, up to 5% GeO.sub.2, V.sub.2 O.sub.5, NiO, CuO, and other transition metal oxides, and up to 2% B.sub.2 O.sub.3 and/or SiO.sub.2.
U.S. Pat. No. 4,456,692 (Dumbaugh, Jr. et al.) records glasses consisting essentially, in weight percent, of
______________________________________ Bi.sub.2 O.sub.3 40-90 Ga.sub.2 O.sub.3 5-30 CdO 0-35 ______________________________________
with up to 30% total of the following components in the indicated proportions of
______________________________________ Cs.sub.2 O 0-10 GeO.sub.2 0-3 HgO 0-25 Sb.sub.2 O.sub.3 0-4. ______________________________________
U.S. Pat. No. 4,483,931 (Dumbaugh, Jr. et al.) discloses glasses consisting essentially, in weight percent, of
______________________________________ Ga.sub.2 O.sub.3 5-30 PbO 10-85 Bi.sub.2 O.sub.3 0-85 ______________________________________
and up to 30% total of the following components in the indicated proportions
______________________________________ Cs.sub.2 O 0-20 Rb.sub.2 O 0-5 MnO.sub.2 0-5 In.sub.2 O.sub.3 0-10 HgO 0-30 HfO.sub.2 0-5 CuO 0-2 SiO.sub.2 0-2 Tl.sub.2 O.sub.3 0-20 Al.sub.2 O.sub.3 0-3 CdO 0-12 ZrO.sub.2 0-5 Sb.sub.2 O.sub.3 0-10 ZnO 0-5 GeO.sub.2 0-5 Nb.sub.2 O.sub.5 0-5 TeO.sub.2 0-10 K.sub.2 O 0-2 Na.sub.2 O 0-2 Ta.sub.2 O.sub.5 0-5. Cr.sub.2 O.sub.3 0-5 ______________________________________
U.S. Pat. No. 5,093,287 (Borrelli et al.) is directed to glasses consisting essentially, in weight percent, of
______________________________________ PbO 42-48 Bi.sub.2 O.sub.3 33-44 Ga.sub.2 O.sub.3 10-15 ______________________________________
and up to 15% total of following components in the indicated proportions of 0.5-5% SiO.sub.2 and/or GeO.sub.2 and 4-15% Tl.sub.2 O.
U.S. Pat. No. 5,093,288 (Aitken et al.) is drawn to glasses consisting essentially, in cation percent, of
______________________________________ TlO.sub.0.5 15-60 GaO.sub.1.5 0.5-10 BiO.sub.1.5 10-45 BiO.sub.1.5 + TlO.sub.0.5 .gtoreq.60 GeO.sub.2 5-40 GaO.sub.1.5 + GeO.sub.2 .gtoreq.15 ______________________________________
and up to 15% total of the following components in the indicated proportions of up to 10% SiO.sub.2 and up to 5% TeO.sub.2.
U.S. Pat. No. 5,114,884 (Lapp et al.), describes glasses consisting essentially, in weight percent, of 7.5-25% Ga.sub.2 O.sub.3, 70-92% Bi.sub.2 O.sub.3, and 0.25-12% R.sub.2 O, wherein R.sub.2 O consists of Na.sub.2 O and/or K.sub.2 O, and wherein up to one-half of the R.sub.2 O may be replaced with an alkali metal halide. Ga.sub.2 O.sub.3 behaves as a glass former in those glasses.
The glasses disclosed in those patents have compositions placing them within a family of glasses frequently termed in the art heavy metal oxide (HMO) glasses. A distinguishing feature of HMO glasses, which glasses can transmit infrared radiations out to wavelengths of 8 microns and longer, resides in the fact that none of the oxides traditionally deemed necessary for stable glass formation, such as B.sub.2 O.sub.3, GeO.sub.2, P.sub.2 O.sub.5, and SiO.sub.2, is present therein. The inclusion of these latter oxides restricts the infrared transmission of conventional glasses to less than about 5.5 microns. Non-oxide glasses which can transmit infrared radiation far into the infrared region of the electromagnetic spectrum, such as halide and chalcogenide glasses, are known in the art, but the application of those glasses has been limited due to complexities encountered in forming the glasses into desired shapes and by their inherent low chemical durability.
The HMO glasses are generally relatively stable and quite easily formed through customary glass forming techniques. Nevertheless, those glasses exhibit several properties which can constrain against their use in traditional optical applications. For example, they exhibit high refractive indices, i.e., a n.sub.D &gt;2 with most values &gt;2.4, which can lead to high reflective losses. They exhibit high linear coefficients of thermal expansion, viz., &gt;100.times.10.sup.-7 /.degree. C. over the temperature range of 25.degree.-250.degree. C., with low annealing points (-300.degree.-375.degree. C.) and strain points (.about.275.degree.-350.degree. C.) which limit use temperatures. They have high densities (.about.8 grams/cm.sup.3) which raise concerns of weight. They frequently are colored, those colors ranging from straw-like to a deep red, a normally undesirable attribute for optical components.
The undesirable characteristics of HMO glasses result from the presence of highly polarizable, weakly bound, heavy metal cations such as lead and bismuth. As was observed above in the discussion of U.S. Pat. No. 5,114,884, Ga.sub.2 O.sub.3 was employed as a glass forming agent. Unfortunately, however, Ga.sub.2 O.sub.3 alone cannot be readily formed in the glassy state. In conventional glass compositions gallium is considered to be an intermediate glass former, i.e., a cation which enhances glass formation, but only when used in conjunction with the standard glass formers. As discussed above, however, the standard glass forming cations are inappropriate for applications where high infrared transmission is desired.
Therefore, the principal objective of the present research was to discover other oxides which, when combined with Ga.sub.2 O.sub.3, would enhance glass formation, but which would not result in a lower fundamental absorption. That is, an additive must not form bonds in the glass which will vibrate at a higher frequency (lower wavelength) than the gallium-oxygen bond. That requirement means that only oxides with cations either more massive than gallium or which result in lower cation-oxygen bond strengths may be utilized.